Copper and copper alloys have been brazed successfully for many years with metals comprising the phosphorus-copper alloys, also known as phos-copper alloys. Silver is often added to the base metals to accomplish special features for a wide variety of applications. These alloys are generally known as silver-phos-copper alloys. Silver brazing alloys, composed primarily of silver, copper, zinc, tin, nickel, manganese, and cadmium, are used to braze ferrous and non-ferrous metals and alloys. These silver brazing alloys are designed to work at low temperatures and to provide strong, ductile joints.
Additions of other elements, such as tin, antimony, nickel or silver, to phos-copper and silver-phos-copper alloys have been made in an effort to lower the melting range temperatures or to reduce the phosphorus content to increase ductility. For example, U.S. Pat. No. 5,066,456 discloses that the addition of tin and antimony up to six percent each to a phos-copper based alloy lowers brazing temperatures.
Air conditioning coils, heat exchangers, water coolers and other copper coils are manufactured by connecting copper tubing and fittings by brazing with phos-copper or silver-phos-copper brazing alloys. These alloys produce strong, ductile brazes, but the industry has long experienced a relatively high percentage of leaks after brazing, particularly with BCuP-2 alloys. Most leaks are caught on the production floor during testing and are repaired. This double work of brazing and testing is very costly. More damaging, very tiny leaks can evade factory testing and end up as warranty work in the field, which is both expensive and damaging to the company's brand image.
The phos-copper alloys now on the market all range within a solidus temperature of 1310° F. to a liquidus temperature of 1500° F. Alloys of even higher phosphorus content, up to 8%, are now in use to enhance productivity because of their lower operating temperature cost considerations. The non-silver alloys in this group are the most commonly used in industry and contain 7% to 7.4% phosphorus, the balance being copper. The fact that these phos-copper alloys flow and join very well is problematic in that they also flow very thinly. Torch and furnace brazing is performed as rapidly as possible to achieve good productivity. While these alloys are quick to braze, they are difficult to observe for soundness. The entire 360° of the brazed joint must be carefully viewed by the operator, for it is here that a correction, if needed, should be made. These thin-flowing alloys produce only a very small cap, or shoulder, around the pipe at the fitting junction. The alloys are thin-flowing in that they flow like a heavy coating of paint, instead of more thickly as in a putty used to seal a ⅛″ crack.
Even a skilled brazer cannot tell 100% of the time that he has a totally leak-free connection by visually looking at his completed braze. In some places on a given braze connection, the brazing alloy can be seen to be in place as a shoulder between the two parts, while in other places the alloy drops in the adjoining area (the capillary) without forming any noticeable shoulder. When viewing this very closely, the operator can often see that the joint appears to be 100% sound, but he can't be certain of it. Most air conditioning companies submerge each copper coil, which comprises perhaps 100 brazes, into a water tank, and air pressure is added to this coil to determine if there are any leaks. Wherever leaks are found, brazing must be repeated.
The now-in-use phos-copper alloys, as described above, could be modified to form an advantageous cap by lowering the phosphorus content significantly. However, doing so is not feasible because the liquidus temperature rises to a point of endangering the copper being brazed. It is noteworthy that silver in the range of 6-15%, when added to the phos-copper alloys described above, lowers the solidus temperature to 1190° F., allows the phosphorus contents to be reduced as much as 2%, allows the alloy to flow in a much thicker manner, and effects a noticeable cap or shoulder to the brazed area. The popular 15% silver-phos-copper alloy has the consistency of hot taffy when hot enough to braze, and easily forms a large cap or shoulder at the joint area. This visible fillet is quickly seen by the operator and any omission in the braze can be remedied. However, the addition of silver is quite expensive.
Another serious deterrent to being able to observe the quality of copper tubing brazed with phos-copper or silver-phos-copper brazing alloys is the formation of a black oxide that is formed on the actual braze surface and on the adjacent copper pipe. Because the braze and the copper pipe all turn black, it is difficult to closely inspect the actual braze.
In addition to the initial soundness of the brazed joint, the corrosion resistance of the joint is of great importance. Brazed parts are used in corrosive environments of varying degree, for example, the marine environment, sewer treatment facilities and underground. The BCuP-5 alloy (80Cu-5P-15Ag) is typically used whenever corrosion is a significant factor. However, further improvement in corrosion resistance is desirable.
There is thus a need for a phos-copper base alloy system that brazes at low temperatures, forms a noticeable cap or shoulder to facilitate visual inspection, does not form black oxide to any extent that will obscure visual inspection, and provides high corrosion resistance.